Method of regenerating spent caustic alkali solution



7, 4 A. P. HEWLETT ETAL 2,428,623 v I 1' METHOD OF REGENERATING SPENT CAUS TIC ALKALI SQLUTION Filed Dec. 22, 1942 J'REATED MAPHTHA 3 meAruvc l2 TOWER I'EEGENERATED.CAOSTIC- J t CA USTIC REG ENERAT OR crunzcs Sol. nus/v 7- OH- Patented Oct. 7, 1947 METHOD OF REGENERATING SPENT CAUSTIC ALKALI SOLUTION Amiot 1P. Hewlett, Cranford, and Henry C. Paulsen, Elizabeth, N. J assignors to Standard Oil Development Company, a corporation of Delaware Application December 22, 1942, Serial No. 469,808

2 Claims.

This invention relates to novel improvements in oil refining and more particularly to an improved method of regenerating treating agents and refining petroleum oils. The invention is particularly adapted for regeneration of spent alkaline solutions used for removing mercaptans and other impurities from petroleum oils as a step in the refining thereof. More specifically, the, invention is particularly concerned with the regeneration of alkali metal hydroxide solutions containing mercaptides formed in the caustic washing operation of petroleum oils, such as naphtha, etc., for the removal of mercaptans,

Plant spent alkaline solutions are usually regenerated by steam distillation at practically atmospheric pressure whereby the mercaptans are liberated from the corresponding mercaptides and the caustic simultaneously concentrated for further use.

The reaction of mercaptans with caustic to form mercaptides is a reversible one which comes to an equilibrium favoring mercaptide formation at low temperature and mercaptan formation at higher temperatures; mercaptan formation is further favored by the removal of mercaptans. A process has now been discovered by which spent alkaline solutions containing mercaptides are regenerated under temperature and pressure conditions suitable to hydrolyze the mercaptides to mercaptans and free sodium hydroxide. In the hydrolyzing chamber the spent caustic solution, preferably preheated, is contacted, preferably by using countercurrent flow, with a hot solvent oil which assists in liberating mercaptans from the mercaptides and simultaneously extracts the liberated mercaptans, thereby also regenerating the caustic soda solution. The solvent oil containing mercaptans is subsequently pumped to a flash still where the mercaptans are distilled off and the solvent oil returned to the hydrolyzing chamupward countercurrent to an aqueous solution of caustic soda which is fed continuously into the tower near the top through line 3. The treating tower 2 may be a plain vertical tank or it may contain bafile plates or may be packed with rings or other suitable packing material designed to facilitate intimate contact between the naphtha and caustic soda. This tower is preferably maintained at a temperature between 30 F. and 140 F., usually between the narrower limits of F. and F. Treated naphtha substantially free from mercaptans is withdrawn continuously from the top of treating tower 2 through line 4. During the contact of the caustic soda with the naphtha in the tower, the caustic soda converts the mercaptans into corresponding sodium mercaptides which due to preferential solubility transfer from the naphtha into the soda solution. The spent caustic soda solution containing mercaptides is continuously removed from the bottom of treating tower 2 through line 5. It is then passed through heat exchanger 0 to be heated, and then passed through line 7 into the regenerating tower 8 near the top thereof. A hydrocarbon solvent oil such as a gas oil boiling between the approximate limits of 500 and 800 F., preferably at least as hot as or hotter than the spent caustic, is passed into regenerating tower 8 near the bottom thereof through line 9 and, due to its lighter gravity, passes upwardly countercurrent to the spent caustic soda solution which is descending from the top of the tower. This regenerating tower is preferably maintained at a temperature above 200 R, such as from 200 to 400 F., preferably 225 to 350 F., and at atmospheric pressure, or preferably at a superatmospheric pressure of at least about 10 lbs. per square inch, such as 10-200 lbs. per square inch, preferably about 15-50 lbs. per square inch. For instance, this regenerating tower may be maintained at a temperature of about 250 F. with a pressure of about 20 lbs. per square inch or as another example at a temperature of about 280 F. with a pressure of about 40 lbs. per square inch.

During the contact of the spent caustic soda with the solvent oil in regenerating tower 8, the mercaptans and other organic impurities removed from the original naphtha by the caustic soda, transfer from the spent caustic soda into the solvent oil by preferential solubility. As a result, therefore, a regenerated caustic soda substantially freed from mercaptides and reinforced in caustic soda concentration is continuously withdrawn from the bottom of regenerating tower 8 amounts of caustic soda which may be lost during the course of normal operation.

The charged solvent oil containing mercaptans and other organic impurities removed from the spent caustic soda in regenerating tower 8 is continuously removed from the top of said tower through line l4 and is passed into a. solvent stripping zone l5 which may conveniently be a still into which steam is passed through line 'IB in order to distill off the dissolved mercaptans and other impurities overhead through line H and are cooled in a suitable cooler 18 and continuously settled into two .layers in suitable separator I9 from which Water is drawn off from the bottom through line 2%! and mercaptans and other organic impurities drawn oii as an upper layer through line 2! in the form of an oily liquid. In the meantime the hot stripped solvent is drawn off continuously from the bottom of the stripping zone l5 through line 22 and passes through heat exchanger 6 in order to heat the spent caustic soda which is also passing therethrough in'indirect heat exchange therewith, and then the stripped solvent is recycled through 'line '9 into the base of the regenerating tower 8. Fresh or make-up solvent oil may be fed into the system through I tercurrently treating a petroleum oil with a'treating agent such as anaqueous caustic soda, continuously regenerating the spent caustic soda by extracting mercaptans therefrom with a solvent oil by 'countercurrent extraction and continuously regenerating the solvent oil by steam stripping the mercaptans and other organic impurities. Efiicient heat economy is obtained by the use of a heat exchanger between the caustic soda regenerator and the stripping zone.

Although the invention has been described as being particularly applicable to the refining of a petroleum naphtha, it may also be used for refining kerosene, mineral seal oil, gas oil or other specific fractions used for Diesel fuel or heating oil purposes.

Instead of using aqueous caustic soda-as the treating agent one may also use other alkali metal hydroxides, such as caustic potash, Generally the concentration of such-caustic solution should be between the approximate limits of 5%- and .35 If desired, one may add to such aqueous alkali solutions a 'solubil-izing agent referred to along with the regeneratedcaustic soda. I he amount of aqueous caustic sodaor otheroil-i-nsoluble treating agent to be used-will, of course, depend upon the amounts :ofimpurities in1the :oil being treated, the viscosity, and other properties of this oil, as well as'the temperature of operation, etc., but will usually be between the approximate limits of 5-100 gallons of caustic soda solution per gallons of naphtha or other petroleum oil being treated.

In adjustin the operation of the regenerating tower, the amount of solvent oil tobe used will usually be about 5-100% by volume of solvent oil in relation to the volume of caustic soda solution being regenerated. The time of contacting in the regenerating tower should be about 1-20 minutes; depending upon the temperature, pressure and amount of impurities in the spent caustic soda'being regenerated, and other operating convditions.

The solvent oil to be used for regenerating the spent caustic soda is preferably a hydrocarbon oil but may-also be any other suitable waterinsoluble liquid which may consist of hydrocarbons, or may consist of oxygenated or other chemical compounds, such as organic esters,- ethers, amines. This solvent may either be a material havin a relatively broad boiling range, such as an Oil havin an initial boiling point of about 500 F. and a final boiling point of about 800 F.,, or a boiling range of 500 to 700 F. or 600 to 800 F., or even narrower boiling range of 500 to 600 F. or 700 to'800 F., or an even narrower boiling range, such as in the case of substantially pure compounds. p

To illustrate how the fundamental principles of the invention may be tested on a small laboratory scale, the following laboratory data are given:

A sample of spent caustic solution (200 'ml.) and 100 'ml. of a heavy mineral white oil, marketed under the trade name Nujol, were charged to a bomb and agitated for five minutes at 200 F. The bomb and contents Were cooled, the contents removed, separated and analyzed- The-results are as follows:

Spent Regenerated Nujol- Caustic Caustic Mercaptan Solution Solution Oil 7 I Expressed as gms. Nabh per liter Caustic'Solution Mercaptans 45:60 38:08 1 7.20 Sodium Sulfide ,8. 00. .8. 32 Free Sodium Hydroxide 5.8. 40 .64.. 96

Actual quantity of mercaptans removed from one liter ofcaustld It is not intended that this inventionbe limited .to the specific modifications shown, nor the specific examples which have been given merely for the sake of illustration, but only :by the appended claims in which it .is intended to .Iclaim all novelty inherent in the invention as well as all modifications coming within the -spiritand scope of the invention.

We claim:

1. The method of regenerating spent caustic alkalisolution .to remove therefrom I-mercaptides resulting from the caustic alkali treatment of naphtha, which comprises contactingsaid spent caustic alkali solution with a petroleum hydrocarbon solvent oil having. a boiling range between the approximate limits of 500 to 800 F., whereby said. mercaptides are extracted from .the spent alkali solution; thereby regeneratingisaid alkali solution, andsepara'tingsaid regenerated alkali solution from the hydrocarbon solvent oil charged with mercaptides extracted from the spent alkali solution. I 1 i v 2. "Process according --to claim 1 "in which the spent caustic alkali solution treated is a spent caustic soda solution, and in which the regeneration of said spent caustic soda with hydrocarbon solvent oil is carried out by continuous countercurrent contacting at a temperature of about 200-400 F. and at a super-atmospheric pressure of about 10 to 200 lbs/sq. in.

AMIOT P. HEWLETT.

HENRY C. PAULSEN.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number 6 UNITED STATES PATENTS Name Date Seeley et a1. June 18, 1940 Casellie et a1 Oct, 7, 1941 Rogers et a1. July 4, 1939 Yabroff et a1. Mar. 28, 1939 Chaney et a1 Apr. 23, 1935 Hancock Dec. 1'7, 1940 Black et a1. July 13, 1926 FOREIGN PATENTS Country Date Great Britain Jan. 23, 1939 

